1. Field of the Invention
The present invention relates to an image transmission device for storing and/or transmitting an image in band-compressed form.
2. Description of the Related Art
An NTSC signal format (having a signal band of approximately 4.2 MHz), based on so-called NTSC (National Television System Committee) standards, has conventionally been used as one signal format for transmitting a multilevel color motion image. In recent years, high-quality television standards such as the high-definition television standards (corresponding to a signal band (video band) of approximately 20 MHz) have been proposed in order to record and reproduce high-fineness, high-resolution images with improved presence.
However, because of the high degree of definition, such a high-definition television signal format must accommodate information the amount of which is several times larger than that of the conventional signal format. Accordingly, if a signal according to the high-definition television signal format is to be stored or transmitted with no original amount of information reduced, it is necessary to prepare an enormously wide storage area or transmission band, and this may impair time, cost or memory savings. Accordingly, it is common practice to store or transmit such a high-definition television signal by means of "band compression", that is, the technique of reducing the amount of information by utilizing either the redundancy of an image or the visual characteristics of human beings. Offset sub-sampling is a representative band-compression technique. In the Off-set sub-sampling, points which are offset as shown in FIGS. 7(B) and 7(C) are alternately sampled as representative values for each frame from an original image signal produced by the 1/2 interlaced scanning as shown in, for example, FIG. 7(A). Then, the sampled values are stored or transmitted as required. In this manner, in the original image signal, odd sample points (o11, o13, . . . o21, o23, . . .) along odd scanning lines and even sample points e12, e14, . . . e22, e24, . . .) along even scanning lines are extracted for transmission, whereby the amount of information can be reduced to 1/2. If a still image is to be reproduced, all the pixels contained in continuous four fields are used to effect inter-field interpolation of the sample points omitted for the transmission. If a motion image is to be reproduced, the pixels in one field only are used to effect inter-line interpolation. By adaptively reproducing the sample points in accordance with variation in motion, it is possible to minimize the deterioration of image quality by utilizing the visual characteristics of human beings.
FIG. 6(A) shows the conventional construction of the encoder used in a typical off-set sub-sampling apparatus.
In the illustrated encoder, an TCI (time compressed integration) encoder 14 is provided with input terminals 11, 12 and 13. A luminance signal Y, a color-difference signal Pr, and a color-difference signal Pb are supplied as a high-definition television image signal to the input terminals 11, 12 and 13, respectively.
The TCI encoder 14 selects the color-difference signals Pr and Pb in alternation every scanning line, applies 1/4 time-compression to the selected signal, and multiplexes it with the luminance signal in a vertical blanking period thereof.
The output from the TCI encoder 14 is input to an inter-field prefilter 15, an intra-field prefilter 16 and a motion vector detecting circuit 17. The inter-field prefilter 15 is a still image area filter utilizing a field memory, and its output is supplied to a mixer 19. The intra-field prefilter 16 is an intra-field two-dimensional filter utilizing a line memory, and the output having passed through a motion image area filter is supplied to the mixer 19 and a motion detecting circuit 18. The motion vector detecting circuit 17 detects one motion vector representative of the whole image and supplies it to a multiplexing circuit 21 as a motion vector signal, thereby preventing the whole image from becoming a blurred moving image area during panning or tilting.
The motion detecting circuit 18 detects the amount of motion on the basis of variation in the luminance (shading) of a pixel of interest, and then adaptively weights the output of the inter-field prefilter 15 and that of the intra-field prefilter 16. The outputs of the respective filters 15 and 16 are mixed in the mixer 19 in accordance with the above weighting. The mixer 19 supplies to a sub-sampling circuit 20 the output mixed at a predetermined ratio. The sub-sampling circuit 20 samples pixels as shown in FIGS. 7(B) and 7(C) and supplies the sampled pixels to the multiplexing circuit 21. The multiplexing circuit 21 provides to an output terminal 22 a multiplexed signal in which the image signal and the motion vector signal are multiplexed with other signals such as audio signals (not shown).
FIG. 6 (B) shows the conventional construction of the decoder used in the off-set sub-sampling apparatus.
The multiplexed signal output from the aforesaid encoder is input to a demultiplexing circuit 24 through a transmission path and the input terminal 23 of the decoder.
The demultiplexing circuit 24 separates the multiplexed signal output from the encoder into the image signals, the motion vector signal, the audio signal (not shown) and the like. The demultiplexing circuit 24 supplies the image signal separated from the aforesaid multiplexed signal to an inter-field interpolation circuit 25, an intra-field interpolation circuit 26 and a motion detecting circuit 27, the motion vector signal to the inter-field interpolation circuit 25, and the audio signal or the like to an audio circuit (not shown).
The inter-field interpolation circuit 25 includes a frame memory and effects inter-field interpolation of a still image area and a process (motion-vector correction) for maintaining the still image area during panning or the like by shifting a readout position in a frame memory. The result is output to a mixer 28. The intra-field interpolation circuit 26 effects line interpolation within a field for the sake of the motion image area and supplies the output to the mixer 28. The motion detecting circuit 27 detects the amount of motion of the pixel of interest and supplies the detected amount of motion to the mixer 28. The mixer 28 mixes the output of the inter-field interpolation circuit 25 and that of the intra-field interpolation circuit 26 at a proportion corresponding to the amount of motion detected by the motion detecting circuit 27, and supplies the mixed output to a TCI decoder 29. The TCI decoder 29 separates the input into the TCI luminance signal Y, the color-difference signal Pr and the color-difference signal Pb, and provides the luminance signal Y, the color-difference signal Pr and the color-difference signal Pb to an output terminal 30, an output terminal 31 and an output terminal 32, respectively.
However, even if the conventional off-set sub-sampling apparatus described above is used for effecting band compression to store or transmit a high-definition television signal, it is still necessary to use a storage area or a transmission band which is several times greater than that required to handle conventional television signals such as NTSC signals. Accordingly, if the future widespread use of high-definition television sets is taken into account, it will be understood that it is costly for individual users to prepare VTRs, image transmission devices or transmission paths as exclusively used for for high-definition television signals.